1. Field of Invention
This invention relates to a barium titanate-based dielectric composition, and more particularly to a barium titanate-based dielectric composition that can be used to form multilayer ceramic chip capacitors having internal base metal electrodes formed of nickel or nickel alloys.
2. Description of Related Art
Multilayer ceramic chip capacitors have been widely utilized as miniature size, high capacitance, high reliability electronic parts. In accordance with increasing demands for smaller, high-performance electronic equipment, multilayer ceramic chip capacitors also have encountered more rigorous demands toward smaller size, higher capacitance, lower cost, and higher reliability.
Multilayer ceramic chip capacitors generally are fabricated by forming alternating layers of an internal electrode forming paste and a dielectric layer-forming paste. Such layers are typically formed by sheeting, printing, or similar techniques, followed by concurrent firing.
Generally, the internal electrodes have been formed of conductors such as Pd and Pd alloys. Although palladium is expensive, it can be partially replaced by the use of relatively inexpensive base metals such as Ni and Ni alloys. The term xe2x80x9cbase metalxe2x80x9d is defined as any metal other than a metal of the precious metal groups (gold, silver, and platinum). Since internal electrodes of base metals become oxidized if fired in ambient air, the dielectric layers and internal electrode layers must be co-fired in a reducing atmosphere. Firing in a reducing atmosphere, however, causes the dielectric layers to be reduced, resulting in a lowering of resistivity. Multilayer ceramic chip capacitors using non-reducible dielectric materials have been proposed; however, such devices typically have a shorter life of insulation resistance (IR) and low reliability.
When the dielectric material is subject to a DC electric field, its relative dielectric constant (K) lowers with time. If thinner dielectric layers are used in order to provide chip capacitors of a smaller size and greater capacitance, application of DC voltages across the capacitor causes the dielectric layers to receive a more intense electric field, resulting in a greater change of dielectric constant (K) with time, that is, a greater change of capacitance with time. Such changes are undesirable in most applications.
Capacitors also are required to have good DC bias performance. The term xe2x80x9cDC bias performancexe2x80x9d (also referred to as the voltage coefficient of capacitance (VCC)) is defined as the rate of change of capacitance with a change in DC bias (voltage). The capacitance generally decreases as the applied DC electric field is increased. Capacitors having poor DC bias performance thus have the problem that when a DC electric field is applied across the capacitors during normal operation, capacitance drops to unacceptable levels.
The Electronic Industry Association (EIA) prescribes a standard for temperature coefficient of capacitance (TCC) known as the X7R characteristic. The X7R characteristic requires that the rate of change of capacitance be within xc2x115% (reference temperature 25xc2x0 C.) over the temperature range xe2x88x9255xc2x0 C. to 125xc2x0 C.
Nomura et al., U.S. Pat. No. 5,335,139, discusses various prior art efforts to construct dielectric compositions. According to Nomura et al., one dielectric material known to meet the X7R characteristic is a composition of the BaTiO3+SrTiO3+MnO. This material, however, is said to experience a great change of capacitance with time under a DC electric field, for example, a capacitance change of xe2x88x9210% to xe2x88x9230% when a DC electric field of 50 volts is applied at 40xc2x0 C. for 1,000 hours. This change fails to meet the X7R characteristic.
Nomura et al. further describe a multilayer ceramic chip capacitor having alternately stacked dielectric layers and internal electrode layers which may be formed of nickel or nickel alloy. In one embodiment, the dielectric layers contain barium titanate as a major component and magnesium oxide, manganese oxide, barium oxide and/or calcium oxide, silicon dioxide, and yttrium oxide as minor components in such proportion that there are present 0.1 to 3 moles of MgO, 0.05 to 1.0 mole of MnO, 2 to 12 moles of BaO+CaO, 2 to 12 moles of SiO2 and up to 1 mole of Y2O3 per 100 moles of BaTiO3. Nomura teaches that samples containing less than 0.1 mole magnesium oxide per 100 moles of BaTiO3 fail to provide the desired temperature dependence of capacitance.
It is desirable to have yttrium oxide present in the ceramic composition to ensure high reliability, provide high resistance to dielectric breakdown, and prevent degradation. However, when yttrium oxide and magnesium oxide are both present in the composition, they tend to interact in a manner that causes the rate of change of capacitance to fall outside of the X7R characteristic at the high end of the temperature range, i.e., 125xc2x0 C. It would thus be desirable to develop a composition that contains yttrium oxide but very low levels of magnesium oxide in order to form capacitors having high reliability and high resistance to DC voltage breakdown while meeting the X7R characteristic at 125xc2x0 C.
The dielectric composition described in Park et al., U.S. Pat. No. 6,185,087, exhibits improved temperature characteristics. However, when it is used in large capacitance multilayer ceramic capacitors where the dielectric thickness is less than eight microns, the rate of capacitance change is out of EIA X7R specifications. Furthermore the addition of 0.1 to 5 mol percent of SiO2 reduces the dielectric constant to less than 3000.
The present invention provides a dielectric composition that can be used to make ceramic multilayer capacitors compatible with nickel and nickel alloys as the internal electrode material. Capacitors may be formed from the dielectric composition of the present invention in a broad range of firing temperatures and atmospheres, and exhibit a high dielectric constant with a small dielectric loss and excellent reliability under highly accelerated life testing conditions. Additionally, the capacitors can have dielectric layers of less than three microns, while exhibiting a low aging rate and superior temperature characteristics that meet the X7R specification of the EIA standard.
The dielectric composition of the present invention comprises a uniform dense microstructure of grains having an average diameter of about 0.4 microns based upon SEM analysis. The grains within the dielectric layers of the capacitors exhibit a type of core-shell structure after firing, wherein the inner core of the fired grains maintains a high dielectric constant. The core-shell structure of the grains is not a traditional core-shell structure, where a defined boundary exists between the core material and the material that comprises the shell. Instead, the material that comprises the shell diffuses into the grains of material comprising the core. The grains of core material comprise a major portion of each dielectric layer, and the shell material diffuses into the spaces left between the grains. A concentration gradient is created within the dielectric layers, so that the concentration of the shell material is highest in the outer portion of the layers and between the grains of core material, and lowest at the dielectric core of the layers.
The dielectric composition of the present invention comprises barium titanate as a major component and manganese oxide, yttrium oxide, holmium oxide, calcium carbonate, silicon oxide, boron oxide, aluminum oxide, magnesium oxide and calcium oxide as minor components. Preferably, such components are present in the dielectric composition as follows: about 99.80 to about 90.00 wt. % BaTiO3; about 0.067 to about 3.364 wt. % Mn3O4; about 0.040 to about 1.976 wt. % Y2O3; about 0.026 to about 1.320 wt. % Ho2O3; about 0.040 to about 1.993 wt. % CaCO3; about 0.011 to about 0.567 wt. % SiO2; about 0.008 to about 0.389 wt. % B2O3; about 0.004 to about 0.179 wt. % Al2O3; about 0.003 to about 0.164 wt. % MgO; and about 0.001 to about 0.047 wt. % CaO.
Multilayer chip capacitors are fabricated by alternately stacking dielectric layers and internal electrodes to form green chips. The compatible internal electrodes inside the multilayer chip capacitors are nickel or nickel alloys. The dielectric composition that forms the dielectric layers is produced by wet milling the components of the dielectric with a binder system. The dielectric composition is placed on a polyester film, coating the film and forming sheets, which are alternately stacked with electrodes to form the green chips.
After the green chips are formed, the binder is removed in air at a temperature less than 300xc2x0 C. Once the binder is removed, the green chips are then fired in a reducing atmosphere of wet nitrogen and hydrogen at the temperature of from about 1200xc2x0 C. to about 1300xc2x0 C. in an atmosphere having oxygen partial pressure 10xe2x88x928 to 10xe2x88x9212 atm. The green chips are then re-oxidized in an atmosphere having oxygen content in the range of 6 to 60 ppm at a temperature xe2x89xa61100xc2x0 C. to enhance insulation resistance of the dielectric.
It has been found that the multilayer ceramic capacitors of the present invention exhibit a very high dielectric constant and perform well in highly accelerated life testing conditions, and also exhibit a high breakdown voltage in a DC field. Under the highly stressed conditions consisting of an applied DC field of 40 volts/xcexcm and. 150xc2x0 C., the ceramic capacitors demonstrate a characteristic life up to 10 hours. The dielectric constant is greater than 3200 and the dissipation factor is less than 3.0% at 25xc2x0 C. when measured at 1 volt RMS. The temperature coefficient of capacitance is within 12% over the temperature range of xe2x88x9255xc2x0 C. to +125xc2x0 C.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.